Ballast with lampholder arc protection

ABSTRACT

The invention provides an electronic ballast for a fluorescent-lamp ( 20 ), including a fluorescent-lamp ballast circuit ( 30 ), an arc detection circuit ( 40 ) electrically connected to the fluorescent-lamp ballast circuit ( 30 ), and a lamp cutoff device ( 50 ) in series with the fluorescent ( 20 ) and electrically coupled to the arc detection circuit ( 40 ). The fluorescent-lamp ballast circuit ( 30 ) provides power to the fluorescent lamp ( 20 ). The lamp cutoff device ( 50 ) is opened when arcing is detected by the arc detection circuit ( 40 ).

This invention relates generally to an electronic ballast for startingand controlling a gas-discharge lamp or a fluorescent lamp, and moreparticularly, to an electronic ballast with arc protection forinstant-start fluorescent lamps.

A fluorescent lamp is a high-efficiency gas discharge lamp that uses anelectric discharge through low-pressure mercury vapor to produceultraviolet (UV) energy. The ultraviolet energy excites phosphorescentmaterials applied as a thin layer on the inside of a glass tube and thephosphors transform the UV to visible light. Ballasts for fluorescentlamps provide high ignition voltages for starting the lamp and controlpower delivery during lamp operation. The ignition voltages ofinstant-start type ballasts may exceed 800 volts peak and as a result,fluorescent lamps are subject to the problem of output arcing. Typicalfluorescent lamps operate with an alternating voltage of several hundredvolts with a frequency usually of more than 30 KHz. Filamentsconstituting electrodes at opposite ends of the lamp alternately serveas electron-emitting cathodes in each frequency cycle.

The instant-start type of fluorescent-lamp ballast is designed to startfluorescent lamps as soon as power is applied. Instant-start circuits,which were originally developed to eliminate separate mechanical starterdevices, are generally more appropriate in lighting applications withlonger burn cycles such as continuous 24-hour operation or with limitedon-and-off switching. Today, T8-style instant-start ballasts are themost popular type of ballast on the market because of their features ofhigh-efficiency, ease of installation, moderate cost, and independentlamp operation, the latter improving system safety by providing lightfrom functioning lamps when an individual lamp ceases operation.Instant-start ballasts require only one pin at each end of thefluorescent lamp, though can be used with lamps having heatablefilaments and two pins at each end if the lamps are rated accordingly bythe lamp manufacturer.

Potential arcing within lampholders of instant-type ballasts is aphenomenon that is being recognized as an undesirable effect to bemitigated. Traditional instant-start ballasts with parallel lampoperation apply a constant high voltage when the output is open and anarc may occur when, for example, an intermittent connection occursbetween the lamp and lamp sockets. It is possible for momentary outputarcing to occur in fluorescent lighting installations when failed lampsare replaced while AC power is applied to the ballast, the arc beingformed between the fixture socket contacts and a pin of the lamp. Arcingmay not only cause degradation of the contacts in the fixture socketsand undue stress on components within the ballast, but also thepotential for overheating of lamp sockets.

In recent years, the lighting industry has been developing technology tosense potential arcing conditions and shut down a lighting system beforearcing becomes a problem. Underwriters Laboratories gives a Class CCrating to ballasts with anti-arcing protection.

Non-arcing cap holders have been designed for fluorescent lamps that canbe used in harsh environmental conditions. Burwell and others disclose alamp holder assembly with waterproof and insulative characteristics in“Non-Arcing Fluorescent Lamp Holder”, U.S. Pat. No. 6,193,534 issuedFeb. 27, 2001. The fluorescent lamp holder assembly is adapted toreceive various injection-molded end cap structures, and in a preferredembodiment includes a fluorescent lamp surrounded by a protectivesleeve. A first end cap covers a first end of the lamp and sleeve, whilea second end cap structure comprising a tube power connector cap coversa second end of the lamp and sleeve. The interior of each cap holder,also referred to as a receptacle, may be shrouded to discourageelectrical arcing and to allow the receptacle to flex. The end caps arepreferably watertight.

While more complex and costly solutions have been used to produceanti-arcing CC ratings for instant-start ballasts, a need exists for aninstant-start CC-rated electronic fluorescent ballast that can beincorporated into existing ballast designs with minimal impact on otherfeatures and functions of the ballast. The improved ballast wouldincorporate good end-of-life and auto-restart features that eliminatethe need to reset power breakers after failed lamps are replaced. Itwould also provide desirable ballast system and method for reducing andpreventing arcing in discharge lamps that are efficient, cost-effectiveand work with most power-supply circuits and fluorescent lamps.

One aspect of the invention is an electronic ballast for a fluorescentlamp, including a fluorescent-lamp ballast circuit, an arc detectioncircuit, and a lamp cutoff device. The fluorescent-lamp ballast circuitprovides power to the fluorescent lamp. The arc detection circuit iselectrically connected to the fluorescent-lamp ballast circuit. The lampcutoff device is in series with the fluorescent lamp and electricallycoupled to the arc detection circuit. When the arc detection circuitdetects arcing, the lamp cutoff device is opened.

Another aspect of the invention is an arc protection circuit for afluorescent lamp, including an arc detection circuit and a lamp cutoffdevice in series with the fluorescent lamp and electrically coupled tothe arc detection circuit. The lamp cutoff device is opened when the arcdetection circuit detects arcing.

Another aspect of the invention is a method of operating a fluorescentlamp with steps to monitor for an arcing condition, and switch off powerto the fluorescent lamp when the arcing condition is detected.

Another aspect of the invention is an arc protection circuit for afluorescent lamp, including an arc detection circuit and means forreducing lamp power supplied to the fluorescent lamp. The arc detectioncircuit is connected in series with the fluorescent lamp and isresponsive to an interruption of current through the fluorescent lamp asan indication of arcing. Lamp power is reduced to the fluorescent lampwhen the interruption of current through the fluorescent lamp isdetected.

The aforementioned, and other features and advantages of the inventionwill become further apparent from the following detailed description ofthe presently preferred embodiments, read in conjunction with theaccompanying drawings. The detailed description and drawings are merelyillustrative of the invention rather than limiting, the scope of theinvention being defined by the appended claims and equivalents thereof.

Various embodiment of the present invention are illustrated by theaccompanying figures, wherein:

FIG. 1 is a block diagram of an electronic ballast for a fluorescentlamp, in accordance with one embodiment of the current invention;

FIG. 2 is a schematic diagram of an arc protection circuit for afluorescent lamp, in accordance with one embodiment of the currentinvention;

FIG. 3 is a timing diagram for an electronic ballast with lampholder arcprotection, in accordance with one embodiment of the current invention;and

FIG. 4 is a flow diagram of a method of operating a fluorescent lamp, inaccordance with one embodiment of the current invention.

FIG. 1 shows a block diagram of an electronic ballast 10 for afluorescent lamp 20, in accordance with one embodiment of the presentinvention. Electronic ballast 10 includes a fluorescent-lamp ballastcircuit 30 and an arc prevention circuit 33 containing an arc detectioncircuit 40 and a lamp cutoff device 50, arc prevention circuit 33 beingelectrically connected in series between fluorescent-lamp ballastcircuit 30 and fluorescent lamp 20. Lamp cutoff device 50 iselectrically coupled to arc detection circuit 40. Electronic ballast 10with arc detection circuit 40 and lamp cutoff device 50 may be used toprovide arc protection for fluorescent lamps operating with instantstart, programmed start, rapid start, and other types of ballastcircuits. Electronic ballast 10 may be connected to one or a set offluorescent lamps 20. In one embodiment, electronic ballast 10 detectsarcing in a lamp socket, takes appropriate action to extinguish the arc,and then switches the lamp power back on after sufficient time haspassed to extinguish the arc.

Fluorescent lamp 20 can have one or two pins at each end; a single pinat each end is needed for instant-start operation, whereas a set of pinsat each end provide for electrical connections to the filaments and forapplication of high voltage across the terminal pins of fluorescent lamp20. Lampholders 22 and 24 are located at each end of fluorescent lamp20, providing electrical contact to the pins at the end of fluorescentlamp 20. Lampholders 22, 24 allow fluorescent lamp 20 to be installedand removed as desired, and provide mechanical support for fluorescentlamp 20. Because the electrical contacts within lampholders 22, 24 canoxidize or loosen, a localized arc between lampholder 22, 24 and thepins at the ends of fluorescent lamp 20 sometimes forms when highvoltage is applied to fluorescent lamp 20.

Fluorescent-lamp ballast circuit 30 provides power to one or morefluorescent lamps 20, starting fluorescent lamps 20 in either aninstant-start or programmed-start mode, and then controlling the powerto sustain the arc with the lamps. As is well known in the art, thistype of circuit comprises an AC-to-DC converter for generating a DCvoltage from an AC line supply 12, and a controlled-frequency invertercircuit to drive the fluorescent lamps at a prescribed voltage andfrequency. The output circuit includes an impedance element to limitcurrent to the lamp. A ballast capacitor 32 is connected between theoutputs of fluorescent-lamp ballast circuit 30 and fluorescent lamp 20to act as an impedance element to limit current to the lamp in theembodiment of FIG. 1.

Arc detection circuit 40 detects arcing that can occur betweenlampholders 22, 24 and fluorescent lamp 20. Arc detection circuit 40detects a lampholder arcing condition such as a lamp overvoltagecondition or an interrupted lamp-current condition. A lamp overvoltagecondition may occur, for example, when an intermittent arc occursbetween fluorescent lamp 20 and lampholder 22, 24 and a voltage isgenerated across the ends of fluorescent lamp 20 that can exceed 600volts or more. An interrupted lamp current condition may occur, forexample, when an intermittent arc occurs and the lamp current isinterrupted. Lamp cutoff device 50 is opened, for example, when arcingis detected with arc detection circuit 40.

A control signal is applied to lamp cutoff device 50 from arc preventioncircuit 33 to allow the lamp to turn on or to keep the lamp turned off.A lamp-on control signal applied to an input control terminal of lampcutoff device 50, for example, turns on lamp cutoff device 50 and allowscurrent to flow, whereas a lamp-off control signal turns off lamp cutoffdevice 50 and blocks current from flowing through fluorescent lamp 20.In one example, lamp cutoff device 50 comprises a high-voltage triac. Inanother example, lamp cutoff device 50 comprises a powermetal-oxide-semiconductor field-effect transistor (MOSFET), a transistorin a diode bridge arrangement, or other power device or power deviceconfiguration. Electronic components such as resistors, capacitors,diodes and transistors are used to bias and provide appropriate signallevels and timing to lamp cutoff device 50. With a triac, for example,high rates of current change (di/dt) occur with large ballast voltagesat relatively high ballast output frequencies across the outputterminals of the triac, maintaining the triac in an on or closedcondition while lamp current continues to flow, acting as a lamp-oncontrol signal. When current is interrupted, the lack of current acts asa lamp-off control signal for triac conduction.

Arc detection and protection may be implemented using other types ofbi-directional switching devices that have circuitry to detect aninterruption of current and/or an increase in lamp voltage above thenormal operating range of the lamp.

In an alternative construction, circuitry maintains connection offluorescent lamp 20 for a predetermined period of time after electronicballast 10 is first turned on before blocking lamp cutoff device 50between fluorescent lamp 20 and fluorescent-lamp ballast circuit 30 toguarantee proper ignition of fluorescent lamp 20.

Arc prevention circuit 33 may contain a lamp cutoff delay circuit 60.Lamp cutoff delay circuit 60 is electrically connected to lamp cutoffdevice 50. Lamp cutoff delay circuit 60 provides a lamp-on controlsignal to lamp cutoff device 50 to maintain lamp cutoff device 50 in aclosed condition during a predetermined startup period. During initialstartup, for example, lamp cutoff delay circuit 60 provides a lamp-oncontrol signal to lamp cutoff device 50 for a period of 100 millisecondsor longer to ensure that fluorescent lamp 20 has a sufficiently longperiod of time to ignite and turn on, even if an intermittent lampholderarc occurs. Lamp cutoff delay circuit 60 may comprise, for example, acharging network, a monostable multivibrator, or a timing chip toachieve the desired delay period.

Arc prevention circuit 33 may also contain a lamp restart circuit 70.Lamp restart circuit 70 is electrically connected to lamp cutoff device50. Lamp restart circuit 70 provides a lamp-on control signal to lampcutoff device 50 when a predetermined lamp restart delay period hasexpired. For example, after an arc has been detected and power tofluorescent lamp 20 has been removed, an attempt may be madeautomatically to restart the lamp. The lamp-restart delay period mayexceed, for example, ten seconds to allow an arc in the lampholder todiminish and any heat generated in the lampholder to dissipate. Thedelay period may be generated, for example, with a timing circuit or atiming chip.

In another embodiment, an arc protection circuit including arc detectioncircuit 40 in series with fluorescent lamp 20 is responsive to aninterruption of current through fluorescent lamp 20 indicating arcing.Lamp power supplied to fluorescent lamp 20 is reduced when theinterruption of current through fluorescent lamp 20 is detected, such asby blocking power to fluorescent lamp 20 with lamp cutoff device 50 whenthe arcing condition is detected or by reducing the voltage applied tofluorescent lamp 20 from fluorescent-lamp ballast circuit 30.

FIG. 2 shows a schematic diagram of an arc protection circuit for afluorescent lamp 20, in accordance with one embodiment of the presentinvention. The arc protection circuit includes a triac 52 functioning asan arc detection circuit 40 and as a lamp cutoff device 50, and a lampcutoff delay circuit 60. When ballast circuit 30 applies power throughballast capacitor 32, it generates a DC voltage across capacitor C2 viacapacitor C1, diode D1, diode D2, and zener diode D4. This DC voltagesupplies current to the gate of triac 52 via resistor R3 rendering itconductive. MOSFET transistor M1 is initially in an off state allowingcurrent to flow. After a predetermined period of time determined by thetime constant of resistor R2 and capacitor C3, the voltage acrosscapacitor C3 reaches the turn-on threshold of transistor M1, turning iton. This removes the gate current from triac 52 and triac 52 will nowonly remain conductive while current continues to flow through it. Whencurrent through triac 52 is interrupted, such as is the case when thereis an arcing occurrence between lamp 20 and one of its lampholders,triac 52 turns off, thereby removing power from the lamp. The lamp-oncontrol signal may be voltage-limited with zener diode D3. Bias resistorR1 limits current through zener diode D3 and the triac gate.

The arc protection circuit includes a lamp cutoff delay circuit 60, asshown back in FIG. 1. Lamp cutoff delay circuit 60 is electricallyconnected to lamp cutoff device 50. Exemplary lamp cutoff delay circuit60 provides a lamp-on control signal to lamp cutoff device 50 tomaintain lamp cutoff device 50 in a closed condition during apredetermined startup period when fluorescent lamp 20 is first beinglit, or during subsequent attempts to be lit after an arc is detectedand power is removed from fluorescent lamp 20. For example, a lamp-oncontrol signal is applied to the gate of a triac or a power MOSFETserving as lamp cutoff device 50 during the startup period whether ornot there is arcing in the lampholder, thereby providing fluorescentlamp 20 adequate time to ignite. In one example, gate current issupplied to the triac when electronic ballast 10 is turned on andcontinues to be supplied for at least a period of several hundreds ofmilliseconds until such time that a high voltage is detected acrossfluorescent lamp 20 and the gate current is removed.

The arc protection circuit may include a lamp restart circuit 70, asshown back in FIG. 1. Lamp restart circuit 70 is electrically connectedto lamp cutoff device 50. Lamp restart circuit 70 provides a lamp-oncontrol signal to lamp cutoff device 50 when a predeterminedlamp-restart delay period has expired. For example, a lamp-off controlsignal is applied to the gate of a triac or a power MOSFET serving aslamp cutoff device 50 during the lamp-restart delay period to allow anyheat generated by an arcing lampholder to dissipate, then a lamp-oncontrol signal is applied after the lamp-restart delay period has beencompleted.

FIG. 3 shows a timing diagram of an arc protection circuit for afluorescent lamp, in accordance with one embodiment of the presentinvention. The timing diagram shows lamp voltages 42 and lamp controlsignals 44 to the lamp cutoff device during characteristic operationalmodes of the invention.

At a time t0, power is applied to the electronic ballast. Ahigh-frequency lamp voltage is applied across the fluorescent lamp, anda lamp-on control signal is applied to a lamp cutoff device in serieswith the fluorescent lamp. At some point during the startup period, thefluorescent lamp is ignited. In the case of an instant-start electronicballast, an elevated voltage is applied to the ends of the fluorescentlamp during the startup period, which is then reduced during normaloperation shown at time t1 when the fluorescent lamp is ignited. The arcdetection circuit monitors for arcing in the lampholders.

When an arc in the lampholder is detected as indicated at time t2, alamp-off control signal is generated and applied to the lamp cutoffdevice, power to the fluorescent lamp is removed, and a lamp-restartdelay period is initiated.

At the end of the lamp-restart delay period, a restart may be attempted.At time t3, a lamp-on control signal is generated and applied to thelamp cutoff device. A ballast voltage is applied to the fluorescentlamp, and the fluorescent lamp is restarted. At the end of a re-startupperiod indicated at time t4, the fluorescent lamp is presumed to haverestarted and normal operation is continued through a time t5 when thepower to the fluorescent-lamp ballast circuit is removed and thefluorescent lamp is turned off.

FIG. 4 shows a flow diagram of a method of operating a fluorescent lamp,in accordance with one embodiment of the present invention.

A startup condition is determined, as seen at block 80. A determinationis made whether a startup condition exists, such as detecting a linevoltage applied to an fluorescent-lamp ballast circuit from the flippingof a wall switch, detecting an occupant in a room and applying linevoltage to the electronic ballast, or cycling on after nighttimeoperation is completed.

When a startup condition is determined, power is applied to thefluorescent lamp, as seen at block 82. During a startup period, aninstant-start electronic ballast may apply an elevated voltage to one ormore fluorescent lamps in a fluorescent lamp fixture until thefluorescent lamps are ignited. Although an arcing condition may bedetected during startup, power is maintained to the fluorescent lamp fora predetermined startup period.

The lamp ignites, as seen at block 84. A discharge along the length ofthe fluorescent lamp is generated within the fluorescent-lamp tube andsustained by continuation of lamp voltage applied to the fluorescentlamp.

While the fluorescent lamp is operating, an arc detection circuitmonitors for an arcing condition, as seen at block 86. Monitoring for anarcing condition may comprise, for example, measuring a fluorescent-lampvoltage and comparing the measured voltage to a threshold value, ormonitoring for an interruption of a fluorescent-lamp current.

When arcing is detected, power to the fluorescent lamp is blocked off,as seen at block 88. Blocking power to the fluorescent lamp maycomprise, for example, sending a lamp-off control signal to a lampcutoff device in series with the fluorescent lamp, or detecting aninterruption in lamp current and turning off a lamp cutoff device inseries with the fluorescent lamp due to reduced di/dt across the lampcutoff device. In some cases, all the power applied to the fluorescentlamps is blocked; in other cases, a significant portion of the power isblocked.

When the lamp current is blocked off, a lamp-restart delay period may beinitiated, as seen at block 90. The lamp-restart delay period may beinitiated, for example, when the arcing condition is detected. Thelamp-restart delay period may be selected to allow time for the arcingto be extinguished and for any heat generated in the vicinity of the arcto be dissipated.

When the lamp-restart delay period is expired, a lamp-on control signalmay be provided to the lamp cutoff device in series with the fluorescentlamp, as seen at block 92. Power is then applied to the fluorescent lampas seen back at block 82, and maintained across the fluorescent lampuntil the lamp re-ignites, as seen at block 84. The steps from block 82through block 92 may be repeated until the fluorescent light is turnedoff.

When power is removed from the fluorescent-lamp ballast circuit, thefluorescent lamp is turned off, as seen at block 94.

While the embodiments of the invention disclosed herein are presentlyconsidered to be preferred, various changes and modifications can bemade without departing from the spirit and scope of the invention. Thescope of the invention is indicated in the appended claims, and allchanges that come within the meaning and range of equivalents areintended to be embraced therein.

1. An electronic ballast for a fluorescent lamp (20), comprising: afluorescent-lamp ballast circuit (30); an arc detection circuit (40)electrically connected to the fluorescent-lamp ballast circuit (30); anda lamp cutoff device (50) in series with the fluorescent lamp (20) andelectrically coupled to the arc detection circuit (40), wherein thefluorescent-lamp ballast circuit (30) provides power to the fluorescentlamp (20), and wherein the lamp cutoff device (50) is opened when arcingis detected by the arc detection circuit (40).
 2. The ballast of claim 1wherein the arc detection circuit (40) detects a lampholder arcingcondition.
 3. The ballast of claim 2 wherein the lampholder arcingcondition comprises one of a lamp overvoltage condition or aninterrupted lamp current condition.
 4. The ballast of claim 1 whereinthe lamp cutoff device (50) comprises a triac.
 5. The ballast of claim 1wherein the lamp cutoff device (50) comprises a powermetal-oxide-semiconductor field-effect transistor.
 6. The ballast ofclaim 1 further comprising: a lamp cutoff delay circuit (60)electrically connected to the lamp cutoff device (50), wherein the lampcutoff delay circuit (60) provides a lamp-on control signal to the lampcutoff device (50) to maintain the lamp cutoff device (50) in a closedcondition during a predetermined startup period.
 7. The ballast of claim1 further comprising: a lamp restart circuit (70) electrically connectedto the lamp cutoff device (50), wherein the lamp restart circuit (70)provides a lamp-on control signal to the lamp cutoff device (50) when apredetermined lamp restart delay period has expired.
 8. An arcprotection circuit for a fluorescent lamp (20), comprising: an arcdetection circuit (40); and a lamp cutoff device (50) in series with thefluorescent lamp (20) and electrically coupled to the arc detectioncircuit (40), wherein the lamp cutoff device (50) is opened when arcingis detected by the arc detection circuit (40).
 9. The arc protectioncircuit of claim 8 wherein the arc detection circuit (40) detects alampholder arcing condition.
 10. The arc protection circuit of claim 9wherein the lampholder arcing condition comprises one of a lampovervoltage condition or an interrupted lamp current condition.
 11. Thearc protection circuit of claim 8 wherein the lamp cutoff device (50)comprises a triac.
 12. The arc protection circuit of claim 8 wherein thelamp cutoff device (50) comprises a power metal-oxide-semiconductorfield-effect transistor.
 13. The arc protection circuit of claim 8further comprising: a lamp cutoff delay circuit (60) electricallyconnected to the lamp cutoff device (50), wherein the lamp cutoff delaycircuit (60) provides a lamp-on control signal to the lamp cutoff device(50) to maintain the lamp cutoff device (50) in a closed conditionduring a predetermined startup period.
 14. The arc protection circuit ofclaim 8 further comprising: a lamp restart circuit (70) electricallyconnected to the lamp cutoff device (50), wherein the lamp restartcircuit (70) provides a lamp-on control signal to the lamp cutoff device(50) when a predetermined lamp restart delay period has expired.
 15. Amethod of operating a fluorescent lamp (20), comprising: monitoring foran arcing condition; and blocking power to the fluorescent lamp (20)when the arcing condition is detected.
 16. The method of claim 15wherein monitoring for an arcing condition comprises measuring afluorescent-lamp voltage and comparing the measured voltage to athreshold value.
 17. The method of claim 15 wherein monitoring for anarcing condition comprises monitoring for an interruption of afluorescent-lamp current.
 18. The method of claim 15 wherein blockingpower to the fluorescent lamp (20) comprises sending a lamp-off controlsignal to a lamp cutoff device (50) in series with the fluorescent lamp(20).
 19. The method of claim 15 further comprising: determining whethera startup condition exists; and maintaining power to the fluorescentlamp (20) for a predetermined startup period based on the determination.20. The method of claim 15 further comprising: initiating a lamp restartdelay period when the arcing condition is detected; and providing alamp-on control signal to a lamp cutoff device (50) in series with thefluorescent lamp (20) when the lamp restart delay period has expired.21. An arc protection circuit for a fluorescent lamp, comprising: an arcdetection circuit (40), wherein the arc detection circuit (40) isconnected in series with the fluorescent lamp (20), and wherein the arcdetection circuit (40) is responsive to an interruption of currentthrough the fluorescent lamp (20) as an indication of arcing; and meansfor reducing lamp power supplied to the fluorescent lamp (20), whereinlamp power is reduced to the fluorescent lamp (20) when the interruptionof current through the fluorescent lamp (20) is detected.